The human eye has a high resolution area called the fovea, which is responsible for the perception of color and fine detail using specialized retinal receptors called cones. The foveal region subtends a visual angle of about a degree of arc. Our perception of the world is therefore time-multiplexed, in that we scan this one degree patch around to fill in the details in which we are interested, and assume that these details don't change while we aren't actively perceiving them with our fovea. Our non-foveal receptors (rods) can detect movement very well; they function to guide our eyes so we can more accurately perceive anything which may have changed since we last looked at it with our fovea.
Because the foveal region is so narrow, and the highest resolution displays are so expensive, many people have speculated about and experimented with displays which attempt to present a high resolution “inset” image which can move inside of a lower resolution “surround” image. Ideally, this high-resolution inset would perfectly follow the movement of the eye so that fovea always perceives it; the image shown by the inset would change to match its current position so the fovea can see the correct image for its position. The lower resolution surround image would either be dimmed or blocked for the area of the surround which is temporarily used for the foveal inset, so as not to distract from it.
There are several technical challenges in producing such a foveal display system. The system must accurately perceive where the viewer is looking, it must compute the correct foveal image for the foveal position, and last but not least, it must move the foveal image to the correct position fast enough for the system to be useful. Faster graphics engines and better gaze tracking systems now available make the first two problems tractable. Using a mechanical means (such as a mirror) to move the foveal inset in a projection system may be feasible with the fastest galvanometers available, but this is expensive and likely to be troublesome given the time response needed.